In normal operation, combined cycle gas turbine power plants generate significant amounts of oxides of nitrogen (NOx) and CO2 as part of the combustion process. In recent years, the abatement of emissions, particularly NOx, has gained increased attention by the public and federal regulatory authorities, such as U.S. Environmental Protection Agency. Thus, significant resources have been dedicated to reducing and/or eliminating such unwanted emissions. In the burning of a hydrocarbon fuel, particularly liquids, the oxides of nitrogen resulting from air fed to the combustor, as well as nitrogen compounds in the fuel itself (such as pyridine), create pollutants that must be reduced in amount or abated before release to the atmosphere.
Gas turbine engines typically operate on what is known as an “open Brayton cycle” in which air is drawn into a compressor to increase the gas pressure and then combusted with a hydrocarbon fuel, typically natural gas, to produce a high temperature working fluid, with the main products of combustion being carbon dioxide, water (steam), free oxygen and nitrogen, together with undesired products such as carbon monoxide, nitrogen oxides and unburned hydrocarbons. The combustion normally takes place under relatively “lean” conditions, i.e., more than the stoichometric amount of oxygen necessary for complete combustion of the hydrocarbon fuel components in order to maintain the combustion temperature below certain practical limits (which, if too high, would adversely affect the cost and durability of materials of construction).
The high temperature, high pressure working fluid from a combustor is fed into the gas turbine engine where the working fluid expands and the gas temperature drops. In most applications, the gas turbine drives the compressor, as well as a generator that generates electric power. In a simple open Brayton cycle, the working fluid leaves the turbine at a relatively high temperature and thus can be used to generate steam in a heat recovery steam generator (“HRSG”) before being exhausted or treated in downstream operations such as for NOx reduction by selective catalytic reduction (“SCR”). The steam created by the heat recovery steam generator can be used as part of a combined cycle plant to drive a steam turbine such as that found in most closed Rankine cycle steam power plants, thereby increasing the power generation efficiency of the entire plant.
One significant deficiency of open Brayton cycle and combined cycle gas turbine systems is that the exhaust gas includes various oxides of nitrogen (NOx) and a significant amount of carbon dioxide (CO2) and carbon monoxide (CO), all of which are now under increasing public scrutiny for possible adverse environmental effects. Thus, various efforts have been made in the past to lower the amount of NOx generated by gas turbine systems before the NOx must be separated and treated. For example, the nominal level of NOx can be reduced by using the exhaust gas from a preliminary combustor (which contains less oxygen and free nitrogen) as the primary source of oxygen available for combustion. See, e.g., U.S. Pat. Nos. 3,792,581 and 4,147,141. Stettler U.S. Pat. No. 3,969,892 similarly discloses a gas turbine system in which a portion of the exhaust gas from the burner is recycled through a heat exchanger and then back into the combustor with a resulting reduction in nitrogen oxide in the exhaust. Lockwood U.S. Pat. No. 3,949,548 discusses an exhaust gas recirculation system in which a portion of the exhaust gas is cooled and recirculated through a compressor, again with a slight expected reduction in nitrogen oxide.
Despite these developments in reducing the amount of NOx constituents present in gas turbine exhaust streams, the need remains for a more efficient and cost-effective method and apparatus for treating the emissions of nitrogen oxides, CO2 and other pollutants, even assuming that their levels in the turbine exhaust can be reduced slightly by conventional means. Past methods of NOx removal in gas turbine systems typically involved one or more of the following processes: SCR, selective noncatalytic reduction, catalytic decomposition or absorption.
SCR processes rely on the selective reduction of NOx using ammonia, with the basic reactions expressed as:4NH3+2NO+2O2→3N2+6H2O; and4NH3+2NO2+O2→3N2+6H2O.
With SCR, the oxides of nitrogen created during combustion can be reduced to acceptable EPA levels. However, such processes suffer from known deficiencies, including the possible formation of other nitrogen-based compounds that require further treatment before being released into the atmosphere. An exhaust stream can be “scrubbed” using processes that convert the NOx to free nitrogen, or that physically separate the NOx from the exhaust. However, such operations tend to decrease the overall efficiency of the gas turbine and fail to initially remove sufficient amounts of NOx from the exhaust stream. Many SCR systems also require heating to maintain a controlled reduction temperature and have a potential for emitting ammonium sulfate.
Prior art selective noncatalytic reduction processes operate without any catalyst to convert the NOx through a reaction with ammonia to nitrogen and water as follows:4NH3+4NO+O2→4N2+6H2O.
Unfortunately, non-catalytic systems tend to be limited by a narrow reaction temperature range and the fact that process temperatures can change with varying gas turbine engine loads. In addition, the process reduces only 60 to 80 percent of the NOx while requiring a large molar volume of NH3.
Catalytic decomposition systems, in addition to being expensive and complex, likewise tend to remove only about 70 percent of the NOx, depending on the effectiveness of the catalyst. A typical decomposition reaction is shown below:
  NO  ⁢          ⁢      →                              ⁢      catalyst      ⁢                            ⁢                    1        2            ⁢              N        2              +                  1        2            ⁢                        O          2                .            
Most absorption processes remove SOx and NOx using an activated char compound. The process is complex, has an NOx removal potential of only about 40 to 60 percent, and requires handling hot solids.
Thus, the existing processes for removing NOx in exhaust streams of gas turbine engines have well-known deficiencies in both cost and effectiveness.
Another major concern in the design and operation of gas turbine power plants is the isolation and efficient removal of carbon dioxide and carbon monoxide. As noted above, large quantities of CO2 are normally produced in combined cycle systems as one of the major products of combustion of natural gas with air. Removing CO2 requires that it first be separated from nitrogen and other gaseous constituents of the working fluid (e.g., by chemical reaction and/or physical absorption). While CO2 sequestration techniques are well-known, significant energy is utilized in separating the CO2 from other constituents such as NOx, and hence the efficiency of the power generation system decreases when such CO2 separation is required. The CO2 can be captured by direct contact between the exhaust gas and an absorbent such as mono-ethanolamine (MEA). However, MEA separation processes can result in significant penalties to the overall efficiency of the plant. State-of-the art amine separation systems invariably have high operational and capital costs, depending on the presence of other compounds in the exhaust stream and the concentration of the CO2 in the flue gas volume.
In recent years, Exhaust Gas Recirculation (EGR) has become a useful technology for increasing the CO2 concentration of the exhaust gas from gas turbine engines, making it easier to isolate the CO2 present in the flue gas. On the other hand, the use of EGR requires a careful balancing of process conditions in order to avoid an increase of other emissions that are environmentally prohibited (including NOx) that can be produced in a low-oxygen environment due to incomplete combustion. EGR levels well below 40% are typically recommended due to the low levels of oxygen present in the combustor. Otherwise, unwanted CO can be produced due to incomplete oxidation to CO2 in rich flames. Similarly, at least some dissociation of CO2 to CO or NO2 to NO can occur in both stoichiometric and “lean” fuel combustions, depending on the specific combustion and EGR conditions involved.